The present invention relates generally to integrated circuits (ICs) and more specifically to the detection and adjustment of signal characteristics.
Integrated circuits (ICs) typically include many switching elements, such as transistors. These switching elements are configured to perform a variety of circuit functions.
The operation of a transistor is typically affected by its process, voltage, and temperature (“PVT”). The “process” component of PVT refers to the process of manufacturing a transistor. The process is often classified as “fast”, “slow”, “nominal”, or anywhere in between. A transistor manufactured using a fast process will transmit signals at a faster rate as compared to a transistor manufactured using a slower process. Likewise, a transistor manufactured using a slow process will transmit signals at a slower rate as compared to a transistor manufactured using a faster process. Once a transistor is manufactured using a particular process, the effect of the process is fixed. Thus, the “process” component of PVT cannot be adjusted to change the operating characteristics of a manufactured transistor.
The “temperature” component of PVT is the temperature at which the transistor operates. Similar to the process used to manufacture a transistor, the temperature at which a transistor operates affects how a transistor operates. In particular, the rate at which a transistor transmits a signal is affected by the temperature at which the transistor operates. For example, a transistor operating at a reference temperature requires a first voltage to transmit signals at a first rate. If the temperature of the transistor decreases, less voltage is needed to transmit signals at the first rate. Similarly, if the temperature of the transistor increases, more voltage is needed to transmit signals at the first rate. The “temperature” component of PVT varies during operation of the transistor. While there is some control over the temperature of an IC, such temperature cannot be sufficiently adjusted to result in a change in its operating characteristics.
The only component of PVT that can be varied effectively during operation to adjust a transistor's characteristics is its voltage. The optimum supply voltage of a transistor varies depending on the transistor's process (e.g., fast or slow) and the transistor's operating temperature. A conventional solution to the variation in the optimum supply voltage is to set the supply voltage to a worst-case value. In transistors manufactured with a fast process or operating at a low temperature, this conventional solution often results in too much power being supplied to a transistor, with the excess power being dissipated.
As an example, if a circuit designer determines (e.g., via simulation of an IC having many transistors) that a transistor manufactured with a slow process needs 3.2 V as a supply voltage, the circuit designer may provide a supply voltage of 3.2 V to each transistor on the IC. If another transistor on the IC was manufactured with a fast process, however, that transistor may only need a supply voltage of 3.0 V. When 3.2 V is supplied, excess power is dissipated on the transistor that only needs 3.0 V as a supply voltage. As the number of transistors on the IC that were manufactured with a fast process (or are operating at a low temperature) increases, the amount of dissipated power increases.
For example, a serializer/deserializer (SerDes) is a circuit that converts parallel data to serial data and vice-versa. FIG. 1 shows an example SerDes circuit 100 having a pre-amplifier 105 in communication with one or more loads (e.g., latches 110-125) over a communication channel 130. The communication channel 130 may be a wire, a backplane, or any medium (e.g., air) that can transmit an output of the pre-amplifier 105 to the latches 110-125. Because the pre-amplifier 105 is transmitting its output signal over communication channel 130, the load (i.e., latches 110-125) seen by the pre-amplifier 105 may not be known. As a result, estimates of the load for fast and slow processes are used in simulation. The simulation often produces a worst-case design of the pre-amplifier 105 for a given load to allow for a load manufactured with a slow process. As stated above, this worst-case design typically results in excess power being dissipated.
FIG. 2 shows a block diagram of a transmitter 205 in communication with a receiver 210 over a communication channel 215. A signal's slew rate, or maximum rate of change with respect to time, typically degrades or changes if not optimized before the signal is transmitted over communication channel 215. This degradation is typically due to nonlinearities and imperfections in the communication channel 215. Further, the signal's swing, or the signal's maximum amplitude, may also degrade as the signal is transmitted over the communication channel 215 if the signal's swing is not optimized for the communication channel 215.
The communication channel is often modeled after making one or more assumptions about the transmitted signal's swing and the transmitted signal's slew rate. As a result, a circuit transmitting a signal via the communication channel operates in an optimum manner as long as the assumptions are correct. Usually, however, the assumptions do not account for secondary effects and, as a result, the circuit does not operate in an optimal manner.
Therefore, there remains a need to detect and correct characteristics (such as slew rate and swing) of a transmitted signal.